1. Field of the Invention
This invention relates to a linear solenoid valve apparatus for changing an output pressure in accordance with an electric signal, and in particular, relates to a linear solenoid valve used for a vehicle hydraulic fluid control device, more particularly, a device for preventing foreign matter from causing valve sticking.
2. Description of the Prior Art
A linear solenoid valve includes a modulator valve and an electromagnetic valve.
The electromagnetic valve has a plunger operated responsive to electric current.
The modulator valve has a spool, a spring biasing the spool, and a feedback chamber, wherein a supply pressure regulated by the modulator valve is supplied from a supply port to an output port, through positioning the spool by balancing with electric energization, spring force and feedback pressure.
Linear solenoid valves used for a vehicle are classified as either the open type in which the exhaust port of the modulator valve is normally open or the closed type in which the port is normally closed.
Furthermore, the closed type is classified as either an underlapped type which connects an output port to both a supply port and an exhaust port, and an Overlapped type which does not connect the output port to either of them.
Since, in the underlapped type, the output port is always connected to the supply port and the exhaust port during regulating an oil pressure, the consumption of the oil is increased and the stability of the oil pressure is lost. It is undesirable to use a linear pressure control device when it is required to provide steady control continuously because of several problems relating to the capacity of the oil pump and the stability of line pressure.
So, in general, the overlapped type linear solenoid valve is preferred for the above described hydraulic control device.